The present invention relates to a network for a passive filter to compensate the variation of the loss attenuation of the filter dependent on ambient temperature variations. More precisely, the invention relates to a network which gives temperature compensation for a filter build up by passive components, the filter showing an operative attenuation characteristic with very high slope at the band limit. Such strong frequency selective filters are, for example, switching filters in carrier frequency equipments for through connection of one or several frequency groups within a basic primary group. The proposed network is, however, also applicable for other types of filters with high frequency selectivity.
The above-mentioned kind of filter can consist of a band pass filter built up by passive LC-elements. At high frequency selectivity, that is, when the losses of the elements, due to the propagation time of the filter cause undesired attenuation at the pass band limits, it is common, either to connect a correction network in order to compensate the characteristics of the operative attenuation at the pass band limit or to compensate the losses of the filter, according to standard method, without an additional correction network. The loss attenuation of the filter changes with the temperature and if the copper losses dominate, the change can reach, for example, 0.4%/.degree. C. This temperature dependence remains in the compensated filter and causes an undesired change of the characteristics of the operative attenuation at the pass band limit.
It is previously known to reduce the undesired change of the loss attenuation by reducing the same, which in practice means a bigger volume of the included coils. In this way, it is possible to make the loss attenuation within a pass band so low that the temperature change will be within the specified tolerances demanded. For filters with very high demands on frequency selectivity these tolerances have, however, in the course of time, been reduced so much that this solution is no longer sufficient. Besides, the known solution implies that space consuming and expensive components must be used.
An object of the present invention is to provide a network for a passive filter, for example, of the band pass character for simple and cheap compensation of the temperature dependent loss attenuation at a chosen band limit of the filter.